Underground workspace - how to shore up and waterproof?

[BodgeBodge]

Hi,

This is probably more of a thought experiment in truth but...

If I wanted to dig a big hole (area of a single garage and 3m deep) and fit a modified car lift (so that I can get one car underground and one car above ground), what would be a typical means of:

 

1) Safely shoring up the sides whilst  excavating.

2) Fitting a structure which will be watertight.

 

My initial thought would be to build a steel box frame and slowly dig / lower it in to place (I.e. it serves as the shoring and the final structure). The inner faces of the frame would then be welded with a skin of sheet steel and then back filled with concrete. I'm sure someone here would have a better idea?

 

Thanks in advance!

 

Si

Edited April 24, 2023 by BodgeBodge

[ProDave]

Leave it for the professionals, you are building a basement and it is serious engineering.

[Iceverge]

It'd be easier to just install a post lift on flat ground and drive another car underneath I'd have thought. 

[Dave Jones]

dig the side trenches first, rebar and conc then digout the middle after

[BodgeBodge]

1 hour ago, ProDave said:

Leave it for the professionals, you are building a basement and it is serious engineering.

And I am a serious (Mechanical, not Civil) Engineer! 😉 Don't worry, as I said, this is more of a thought experiment...

[BodgeBodge]

30 minutes ago, Iceverge said:

It'd be easier to just install a post lift on flat ground and drive another car underneath I'd have thought. 

You are right but unfortunately, it would not be possible to build a structure that tall in this location

[BodgeBodge]

29 minutes ago, Dave Jones said:

dig the side trenches first, rebar and conc then digout the middle after

Interesting. Would it be possible to dig a trench that deep if it were so narrow? Specialist machinery required. Also, it would surely still need shoring up.

[markc]

2 hours ago, BodgeBodge said:

Interesting. Would it be possible to dig a trench that deep if it were so narrow? Specialist machinery required. Also, it would surely still need shoring up.

Shoring depends on the ground, deep narrow excavations are often done as casements, if the soil won’t support then the hole is kept full of slurry to resist collapse. In the past shafts we’re sunk in poor ground by digging a shallow hole, inserting a “cutter ring” made of steel or cast iron that sinks as the soil is excavated, masonary walls are then built upwards (remaining around ground level) as the whole thing sinks deeper.

[saveasteading]

For lowest cost, wait for a very dry summer so you can dig a big hole. Then build it.

Otherwise you are looking at coffer dams: sheet piling that will allow you to dig.

 

Then build a waterproof floor and walls: an inside out swimming pool.

So you line the underside and outside. The concrete has to not crack much.

 

Last one I did we used a bentonite membrane. It self heals if it is punctured or torn.

 

Then either cover, or install a sump and pump.

 

So your £15,000 car hoist will need a £75,000 hole in the ground.

Even in central London that might not be worth doing.

[BodgeBodge]

8 hours ago, markc said:

Shoring depends on the ground, deep narrow excavations are often done as casements, if the soil won’t support then the hole is kept full of slurry to resist collapse. In the past shafts we’re sunk in poor ground by digging a shallow hole, inserting a “cutter ring” made of steel or cast iron that sinks as the soil is excavated, masonary walls are then built upwards (remaining around ground level) as the whole thing sinks deeper.

The cutter ring method sounds interesting - out of curiosity, do you know how would that be waterproofed? Tanking of some kind on the inner face perhaps?

[BodgeBodge]

7 hours ago, saveasteading said:

For lowest cost, wait for a very dry summer so you can dig a big hole. Then build it.

Otherwise you are looking at coffer dams: sheet piling that will allow you to dig.

 

Then build a waterproof floor and walls: an inside out swimming pool.

So you line the underside and outside. The concrete has to not crack much.

 

Last one I did we used a bentonite membrane. It self heals if it is punctured or torn.

 

Then either cover, or install a sump and pump.

 

So your £15,000 car hoist will need a £75,000 hole in the ground.

Even in central London that might not be worth doing.

Thank you - this is good info. I had not heard of Bentonite. Will make a study of swimming pool construction.

[Dave Jones]

11 hours ago, BodgeBodge said:

Interesting. Would it be possible to dig a trench that deep if it were so narrow? Specialist machinery required. Also, it would surely still need shoring up.

 

a 3 ton digger can reach 3m deep. Hire in a trench box if the ground is crap. couple days work for a man and a digger.

[BodgeBodge]

15 hours ago, Dave Jones said:

 

a 3 ton digger can reach 3m deep. Hire in a trench box if the ground is crap. couple days work for a man and a digger.

Thanks for the info - will have a look into it! 

[saveasteading]

16 hours ago, BodgeBodge said:

had not heard of Bentonite. Will make a study of swimming pool construction.

Ahhh. 2 different things, sorry.

 

I meant the principle of keeping water in rather than out.

 

A swimming pool is tiled of vinyl lined.

Bentonite is a very heavy mineral, mixed as a dust in water to fill trenches in very heavy Civil Engineering, but I came across it in membrane form. No issues with the basement after 8 years.

[BodgeBodge]

22 hours ago, saveasteading said:

Ahhh. 2 different things, sorry.

 

I meant the principle of keeping water in rather than out.

 

A swimming pool is tiled of vinyl lined.

Bentonite is a very heavy mineral, mixed as a dust in water to fill trenches in very heavy Civil Engineering, but I came across it in membrane form. No issues with the basement after 8 years.

Yes - understood. Thank you. I did a Google of Bentonite membranes and it came up with some interesting products; "self swelling, self healing waterproofing membrane.

The bentonite used in its manufacture is capable of repeated cycles of wetting and drying without deterioration of its sealing properties and has a life expectancy measured in thousands of years (BBA Certified)." 😀

[saveasteading]

We built the walls using a special hollow concrete block. A bit like the eps superstructure systems but stronger. This did away with special shuttering for concrete, and so could use our usual groundworkers. Reinforcement ran horiz and vertical, then  concrete filled.

Many years ago so would need checking out for new methods  if any.

[Big Jimbo]

I like the idea of a great big one piece pond liner type affair, and build inside it. If the water table was below the bottom you could prob get away with it. It the water table was high though, the pressure trying to push the building in would be immense. Years ago had a mate who did a similar thing. Had a great big hole. Concrete base, with engineering brick walls. He did something like boat building for a living, so bought a load of fiberglass home, and did the inside including the floor. It was dry as a bone, and remained so for atleast 10 years while he lived there.

[BodgeBodge]

13 hours ago, Big Jimbo said:

I like the idea of a great big one piece pond liner type affair, and build inside it. If the water table was below the bottom you could prob get away with it. It the water table was high though, the pressure trying to push the building in would be immense. Years ago had a mate who did a similar thing. Had a great big hole. Concrete base, with engineering brick walls. He did something like boat building for a living, so bought a load of fiberglass home, and did the inside including the floor. It was dry as a bone, and remained so for atleast 10 years while he lived there.

Very cool - I like the sound of your friends project! I was also thinking about the possibility of a big liner - surely the hydrostatic pressure wouldn't be very high? Even if the water table was right up to surface (which it isnt), is the hydrostatic pressure at 3m below surface still relatively low (somewhere around 4.3psi)? Easily enough to get through porous material but surely not through a decent (intact) liner / membrane of some kind? This isn't a topic I know much about so am happy to be corrected!

[BodgeBodge]

15 hours ago, saveasteading said:

We built the walls using a special hollow concrete block. A bit like the eps superstructure systems but stronger. This did away with special shuttering for concrete, and so could use our usual groundworkers. Reinforcement ran horiz and vertical, then  concrete filled.

Many years ago so would need checking out for new methods  if any.

Thanks! I assume the vertical reinforcement is some kind of rod which runs through the centre of the block? How did the horizontal reinforcements work? Was it a special type of concrete do you recall?

Edited April 27, 2023 by BodgeBodge

[Big Jimbo]

There are also liquid rubber products now. Might be worth considering. Big pond liner on the outside, liquid rubber on the inside.

[Kelvin]

On 25/04/2023 at 23:12, saveasteading said:

Ahhh. 2 different things, sorry.

 

I meant the principle of keeping water in rather than out.

 

A swimming pool is tiled of vinyl lined.

Bentonite is a very heavy mineral, mixed as a dust in water to fill trenches in very heavy Civil Engineering, but I came across it in membrane form. No issues with the basement after 8 years.

Also used to backfill redundant boreholes 

[TonyT]

Sika additives into concrete/render/

French drains

tanking to external

relandscaping to ensure water stays away from building as much as possible

[Temp]

I have a feeling HMRC might refuse the VAT reclaim on the grounds that car lifts aren't normally provided with houses?

[saveasteading]

22 hours ago, BodgeBodge said:

How did the horizontal reinforcements work

The blocks had slots for the rods. The blocks sat dry in each other and It joggled a bit.

The concrete was 20mm down aggregate. Not really special.

No idea if still available but it  should be as it worked well.

There was no need  for any internal lining other than builders' work.

[Iceverge]

 

As an off the wall idea you could use a precast box culvert turned on its side.

 

Either dig a hole and drop it into it or.......

 

And I like this idea....

 

Lay it the ground and start digging the soil out from around it, gradually allowing it to sink into the earth until the desired level is reached. Using the culvert as a trench box shutter should be pretty safe too. 

 

Pour a concrete floor with a sump and pump and you're done.